 export default class TreeNode {
          val: number
          left: TreeNode | null
          right: TreeNode | null
          constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
              this.val = (val===undefined ? 0 : val)
              this.left = (left===undefined ? null : left)
              this.right = (right===undefined ? null : right)
          }
      }